Photovoltaic devices are known which use a substantially intrinsic (i-type) amorphous semiconductor as a photoactive layer which contributes to a photoelectric conversion. The photovoltaic layer is obtained by decomposition of a source gas such as SiH.sub.4 or Si.sub.2 H.sub.6 containing elemental Si. Such amorphous semiconductor unavoidably contains hydrogen from the source gas and the hydrogen serves as a terminator of dangling bonds in the semiconductor network.
An article by Y. Kuwano et al. in J. Non-Cryst. Solids, Vols. 97 and 98, 1987, pp. 289-292 teaches that if the quantity of the Si-H.sub.2 bond density in the photoactive layer is decreased, it is possible to improve the photoelectric conversion efficiency of the device for a long period. In other words, so-called light induced degradation is reduced. If the concentration of hydrogen in the photoactive layer is decreased, the optical energy band gap Egopt becomes narrow. Accordingly, in a photoactive layer of hydrogenated amorphous silicon (a--Si:H) for example, the absorption coefficient for light in a long wavelength region of about 600 nm or greater, increases and thus it is possible to provide a photovoltaic device having an increased photoelectric current.
It is known that the hydrogen concentration in a deposited amorphous semiconductor film becomes lower if the amorphous semiconductor film is deposited on a substrate having a higher temperature. However, in the case of manufacturing a photovoltaic device, an i-type photoactive layer is normally deposited on a p-type or n-type doped layer already formed on a substrate. Thus the photoactive layer is not directly deposited on the substrate. If the temperature of the substrate is excessively high when a photoactive layer is deposited on a doped layer, dopant or hydrogen in the doped layer would be dissipated or diffused into the photoactive layer which is being grown.
If dopant or hydrogen is diffused into the i-type photoactive layer from the doped layer, the film quality of the i-type layer is impaired and the optical energy band structure changes, particularly near the interface between the doped layer and the i-type layer, whereby the output characteristics of the photovoltaic device are also impaired.
FIG. 1 shows schematically an example of an energy band in a semiconductor film structure having a p-type layer, an i-type layer, and an n-type layer deposited on a substrate in the stated order. In this graph, the ordinate represents the energy band. The dash-dotted horizontal line F represents the Fermi level. The solid line curve A on the upper side represents a lower limit of the conduction band in a case where the p-type layer, the i-type layer and the n type layer are deposited at a relatively low temperature. The solid line curve B on the lower side represents an upper limit of the valence band. If the i-type layer is deposited on the p-type layer at a high temperature, dopant or hydrogen is dissipated from the p-type layer or diffused into the i-type layer and accordingly the energy band structure changes as shown by the broken line curves. More specifically, the energy difference between the p-type layer and the i-type layer becomes small and particularly the inclination of the energy band near the interface between the p-type layer and the i-type layer becomes flat. Consequently, the internal electric field for drifting carriers generated in the i-type layer by light irradiation becomes weak and particularly the carriers are likely to be trapped in a region near the interface between the i-type layer and the p-type layer. As a result, the output characteristics of the photovoltaic device deteriorate.
In addition, a doped layer on the side of incidence of light has preferably a so-called window effect whereby the amount of incident light absorbed in the doped layer is reduced. Therefore, the doped layer on the side of light incidence is intentionally formed to contain 15% hydrogen or more, for example, so that it has a large optical energy band gap. Therefore, if a photoactive layer is to be deposited on a doped layer on the side of light incidence, it is desired to deposit the photoactive layer at a relatively low temperature in order to avoid dissipation or diffusion of hydrogen.
As described above, for reducing the light induced degradation of the photovoltaic device, it is desirable to deposit the photoactive layer at a relatively high temperature. However, for the purpose of preventing a deterioration of the output characteristics, it is desirable to deposit the photoactive layer at a relatively low temperature of 200.degree. C. or less. Hence, these two requirement pose a problem.